1. Field of the Invention
The present invention generally relates to the management of a radio transmission system and the monitoring of its performances. In particular, it concerns a method for rendering the signalling of the radio link performances independent of the source degrading the Quality of Service thereof, and for discriminating the type of noise which the radio link is affected by.
2. Description of the Prior Art
The channel corrupting effects, in the domain of a high-capacity radio transmission system, are notoriously distinguished between selective fading and flat fading. The first distorting phenomenon is due to multipaths: the receive antenna can indeed receive, together with desired signal, a delayed replica thereof, caused by the transmitted signal reflection against trophospheric layers or by reflection against orographic obstacles. The desired signal may also become attenuated, with consequent increase of white noise, this latter phenomenon being identified by the cited second corrupting effect.
Under particularly unfavourable conditions, the fading can lead the radio system to outage conditions, rendering the received signal, even processed and/or properly equalized by the receiver system, no longer intelligible.
In order to overcome possible outage situations of the transmission system, the radio systems may be provided with protection mechanisms. For instance, 1+1 configurations are available, wherein the transmission system to be protected (main) is juxtaposed to a spare (stand-by) channel devoted to come into operation if the Quality of Service of the first one is about to become unreliable.
In general, in a N+1 configuration, the spare channel replaces the first of the N protected channels of which the signalling system announces an outage situation.
Thus the importance arises of monitoring the performances of each radio link in a reliable manner in order to evaluate the Quality of Service level and possibly to operatively act through an exchange (if present in the transmission system) in order to improve it. It is however pointed out that the knowledge of the quality level of the radio link (performance monitoring) is a useful information per se, apart from the possible presence of exchange mechanisms.
At present, all the current performance monitoring methods of a radio system monitor the signal and calculate a numeric quantity that is compared with an acceptable limit value or threshold value: if the calculated number is greater that the threshold, it is believe that the transmission is becoming as degraded and, should the above mentioned exchange mechanisms be provided, the transmission is passed to the spare channel.
However, the inefficiency of such methods arises from the fact that the calculated values corresponding for instance to a BER of 10−9, value that can be considered indicative of a beginning of degrade, are different if we are in the presence of flat fading or selective fading. Unfortunately, at present it is not possible to know if we are really in the presence of the one or the other distorting phenomenon and therefore a compromise is carried out, namely it is decided to forewarn the system at a certain value. The drawback of this procedure is that one is forced either to tolerate a channel degraded by an excessive selective fading or, conversely, to forewarn the system in the presence of a still-acceptable flat fading.